312 ANALYTICAL MIOCI IAN ICS the moment of inertia of the pendulum equals ml?. Therefore substituting this value of / in the expressions for /\> and P and replacing D by I we obtain ™ my I) (XV) for the first approximation, and for the second approximation. 243. Equivalent Simple Pendulum.. — A simple pendulum which has the same period as a physical pendulum is (tailed the equivalent simple pendulum of the latter. If / denotes the length of the equivalent simple pendulum, then a y> ft"2 J~ 7)2 ., Z_A_±«-. (XVI) For a given value of D and a given direction of the axis, K is constant. Therefore if the direction of the axis is not •changed I is a function of I) alone. If we plot the last equation with I as ordinate and I) as abscissa we obtain a curve •similar to that of Fig. 139. It is evident from the curve that the value of I is infinitely large for I) = 0, but it diminishes rapidly to the minimum value— as I) reaches the value A". As D is increased further I increases continually. It will be ob- r> rr served that for a given value of I greater than there are two values of I), one of which is less and the other greater than K.